This invention relates to a direct-coupling control system for a torque converter in an automatic transmission for vehicles, which automatic transmission is provided with a torque converter having an input member including a pump wheel, and an output member including a turbine wheel; an auxiliary transmission having one or a plurality of gear trains and adapted to transmit the torque of the output member to driving wheels via the gear trains; and a direct-coupling clutch disposed between the input and output members, capable of mechanically connecting the input and output members together, which can be permitted to slip. The control system is characterized in that the direct-coupling clutch is operated under predetermined conditions to connect the input and output members mechanically and thereby minimize slipping losses of the torque converter.
When a torque converter is put in a direct-coupled state by an operation of a direct-coupling clutch, fluid slipping of the torque converter does not occur. This contributes greatly to the fuel economy and reduction of generation of sounds of a vehicle during the cruising. However, when a direct-coupling clutch is operated when the speed of the vehicle is extremely low, the following problems can occur.
A first problem is that, even when the accelerator pedal is stepped on during the low-speed cruising of the vehicle to attempt to accelerate, or even when the accelerator pedal is stepped on at a starting end of a slope to attempt to increase the level of engine output, sufficient acceleration performance of the torque converter cannot be obtained since the torque-amplifying effect, which is a primary operation of the torque converter, and which is carried out on the basis of the fluid transmission thereof, is suppressed while the torque converter is in a direct-coupled state. Therefore, when the frequency of stepping on accelerator pedal becomes excessively high, kick-down is made repeatedly in an auxiliary transmission. This troubles a driver much and prevents the attainment of intended reduction of fuel cost.
This kind of problem would be solved by providing a means for detecting the degree of opening of a throttle in an engine and interrupting an operation of the direct-coupling clutch whenever the detected degree of opening of the throttle is not less than a predetermined level. However, when this technique is employed, the user has to accept the consequence of the necessary provision of a complicated control system, and a decrease of reduction of fuel cost. The amount of loss of fuel swings increases in proportion to the frequency of the interruption of the operation of the direct-coupling clutch.
A second problem resides in the following. Starting operation of a direct-coupling clutch in a low-speed region means that the direct coupling of a torque converter is also started in a low-speed region in which the number of revolutions per minute of an engine can be, for example, as low as 1500. In such a low-revolution region, the torque of the engine varies greatly. The time intervals between explosions are comparatively long, so that the vehicle body is vibrated greatly.
When the frequency of such vibrations agree with a resonance frequency of a support system for a power plant as a whole including the engine, the vibrations are transmitted to the passenger compartment to cause contained sounds to be produced. This brings about a result opposite to what is intended to attain in the invention, i.e. the reduction of sounds of a vehicle.
A third problem resides in that, when the engine is accelerated, the engine torque varies greatly, and a power plant as a whole shakes due to its counter-torque, so that the driving sense is lost. In order to solve this problem, an operation of a direct-coupling clutch may be interrupted when a detected moving speed of an accelerator pedal is not less than a predetermined level. However, employing such a countermeasure generally requires precise electronic techniques. This necessarily causes an increase in the manufacturing cost.